Projection Fibres of the Cerebro-Spinal Axis

The projection fibres connect the cerebrum and cerebellum with the rest of the brain and with the spinal cord. They are of two kinds-descending and ascending.

(a) Descending. - The principal descending fibres originate as axons of the pyramidal motor cells of the cerebral cortex. From their origins they converge in the corona radiata to the genu and posterior limb of the internal capsule, and run down through the crustae of the mid-brain. Those fibres which traverse the genu are termed geniculate, the remainder are the pyramidal fibres (Fig. 66).

The geniculate fibres cross the middle line and arborise around the cells of the nuclei of the motor cerebral nerves of the opposite side of the brain. The pyramidal fibres continue through the pons into the pyramids of the medulla. At the lower end of the medulla they undergo a partial crossing or decussation. The inner fibres, comprising about 75% of the whole, cross to the opposite side and run down in the lateral column of the spinal cord as the crossed pyramidal tract.

From this tract, fibres are given off in each segment of the cord to terminate around cells in the posterior horn; the axons of these latter cells arborise in turn around the large multipolar cells of the anterior horn whence the anterior root fibres are derived.

The outermost fibres of the pyramids descend without decussation in the ,nterior column of the spinal cord as the direct pyramidal tract. In each egment of the spinal cord this tract gives off fibres which cross to the opposite ide and terminate around cells of the posterior horn. These, again, communicate with the motor cells of the anterior horn.

It will be seen that all these motor fibres coming down from the cerebral hemispheres cross to the side opposite to that from which they are derived. If, therefore, their areas of origin on one cerebral hemisphere be stimulated, muscular movements of the other side of the body will be evoked.

In animals, stimulation of one side of the cerebral cortex is followed by a certain amount of muscular movement on the same side of the body, indicating the presence of subsidiary fibres or tracts which do not cross. In man, however, where the pyramidal system is most completely developed, the number of these other fibres is extremely limited.

Other descending fibres from the cerebral cortex run with the pyramidal and geniculate fibres through the crusta. They terminate round the smal nuclei pontis; round these same nuclei collaterals from the pyramidal and geclicizlate fibres also arborise. From the cells of the pontine nuclei new fibres originate and pass either directly or after decussation to the cerebellum by way of its middle peduncles (Fig. 67).

A descending system of fibres from the cerebral cortex runs to the thalamus. New fibres are carried thence, either directly or after interruption in the red nucleus, to the cerebellum through the superior peduncle. The descending projection fibres from the cerebellum to the spinal cord follow two main routes.

One group runs out through the superior peduncle, crosses to the opposite side, and terminates round cells of the red nucleus in the tegmentum; from the red nucleus fibres pass down into the spinal cord. The other set emerges by the inferior peduncle and enters the anterior and lateral columns of the spinal cord (Fig. 68).

(b) Ascending. - The ascending projection fibres enter the spinal cord and brain in the first instance from the ganglia on the sensory nerves. As already pointed out the main portions of the entering axons ascend in the cerebro­spinal axis, but large descending branches are also given off.

The majority of the axons of the spinal ganglia run up in the columns of Goll and Burdach to the medulla where they end by arborising around the cells of the gracile and cuneate nuclei. From these nuclei new axons arise, cross the middle line and traverse the opposite side of the brain as the mesial fillet (Fig. 69).

The entering axons from the ganglia on the sensory cerebral nerves terminate round the cells of the sensory nuclei in the medulla and pons; the axons of the cells of these sensory nuclei cross the middle line and join the mesial fillet of the opposite side.

The mesial fillet can be traced through the tegmentum to the thalamus in which most of its fibres end; from the cells of the thalamus fibres radiate to all parts of the cerebral cortex. Some fibres of the mesial fillet pass directly to the cerebral cortex without interruption in the thalamus.

All the axons of the spinal ganglia, however, do not run up to the medulla in the columns of Goll and Burdach. Some proceed at once into the grey matter of the spinal cord. Their further course is extremely problematical, but apparently impulses communicated through them are carried in the antero lateral basis bundles by various relays of intermediate neurons to the upper part of the spinal cord.

Some of these intermediate neurons cross to the opposite side, and others are uncrossed. The antero-lateral basis bundles are continued into the hind-brain as the posterior longitudinal fasciculi which connect up the various nuclei of the cerebral nerves and ultimately reach the thalamus.

A third group of the axons of the spinal ganglia run upwards for a short distance in the column of Burdach and then leave it to arborise round the cells of Clarke's column in the posterior horn. The fibres from. Clarke's cells form the direct cerebellar tract which runs up in the inferior peduncle and terminates in the cerebellum. The other well defined ascending projection fibres which enter the cerebellum are:

(1) fibres from the spinal cord, entering by the inferior and superior peduncles;

(2) fibres from the gracile, cuneate, and inferior olivary nuclei of the medulla, both of the same and of the opposite side; these enter by the inferior peduncle;

(3) fibres from the nuclei of the sensory cerebral nerves, also running in through the inferior peduncle.

The only other tract ascending to the cerebral cortex which need be mentioned here is that which originates from the cortex and nuclei of the cerebellum and proceeds upwards in the superior cerebellar peduncle.

Many of its fibres terminate directly in the thalamus of the opposite side but some are interrupted in the red nucleus, new axons carrying their impulses thence to the thalamus. From the thalamus radiating fibres run to the cerebral cortex (Fig. 70).